Star capacitor units

For ungrounded star capacitor units In this case when it causes a line to neutral fault the impedance of the other two phases will limit the fault current to almost three times the rated current. It is still easy to choose HRC fuses so that they would reinain inoperative... 

Figure 23.5 Build-up of capacitive voltage (TRV) on restrike during contact interruption in a grounded star HT capacitor unit...

It may be observed that when the number of units in parallel are fewer than six, the voltage across the healthy units will rise by more than 10%, even on a failure of only one unit. The units in parallel therefore must be a certain minimum number to ensure that in no case will the voltage across the healthy units in the event of a failure of a few units exceed 10%. Single-star capacitor banks (Figure 25.5(a))... 

For-della connected capacitor units In (his case, it will establish a line-to-line fault with a heavy fault current. Protection by HRC fuses would be more appropriate than for a grounded star or using an MCCB. For series-parallel connected units I his is applicable in HT capacitor banks comprising a number of small units arranged in series and parallel combinations. The fault current in such cases for an isolated neutral can be expressed by... 

The lest voltage should be 2.5 V, irrespective of star- or delta-connected units. Tlie capacitor unit should be charged with the test voltage and discharged five times within 10 minutes. Within 5 minutes after the test the unit may be subjected to voltage lest (.3) between the terminals. The... 

Shunt capacitors 1120 kVAr in each arm of a double star as shown below. Each unit 80 kVAr... 

For smaller units, they are u.sed to limit only the inrush currents during a parallel operation and may be rated for 0.2% of the capacitors kVAr rating. They must be connected on the neutral side of a star-connected capacitor bank. Also refer to the notes below step 4 of Example 23.4. 

This method is applicable to single-star or delta-connected capacitor banks. Unbalance can be detected through the use of an RVT (residual voltage transformer) (Section 15.4.3). See Figure 26.4. The theory of operation is that any unbalance, of the system or the capacitor bank, will shift the neutral and reflect as the residual voltage across the open delta and can be used for the protective scheme. The unbalance voltage across the open delta in the event of failure of a unit in any series group can be expressed by... 

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